The effect of the addition of thiol reagents to heated milk on protein interactions and acid gelation properties

Low levels of β-mercaptoethanol (β-ME) or N-ethylmaleimide (NEM) were added to preheated skim milk (SM) and preheated whey protein-enriched skim milk (WPE-SM). Addition of NEM did not affect the heat-induced interactions between the proteins in heated SM and WPE-SM; addition of β-ME reduced most disulphide bonds of κ-casein. Neither NEM nor β-ME affected the distribution of proteins between the colloidal and serum phases. The heated then treated SM and WPE-SM were acidified to form acid gels. Acid gels containing β-ME had higher yield stress and G′ values than those made from control heated milk. In contrast, adding NEM to heated SM and WPE-SM lowered the yield stress values of the acid gels, but affected the final G′ values only slightly. The rheological results suggested that thiol-disulphide exchange reactions occurred during acid gelation. However, the newly formed disulphide bonds influenced only the yield properties, not the G′ values of the gels.